Several applications exist in which it is necessary to cool the metal body of a machine, device, component, or other type of heat exchanger during its operation to maintain its temperature below a predetermined threshold. For instance, the body of a turbomolecular vacuum pump, typically made of aluminum or aluminum alloys, is constantly cooled during the pump operation to maintain its temperature below a predetermined threshold.
Different methods are known for cooling the aluminum body of a turbomolecular vacuum pump by using low temperature water flowing in the body. In certain prior art applications, the channels through which the cooling fluid flows are directly formed in the pump body by drilling rectilinear (i.e., straight and/or perpendicular) branches connected in series and, at the outer surface of the body, the channels are connected to a circuit of the cooling water by means of inlet and outlet hydraulic connectors made of stainless steel. Even though such a solution is economical and avoids introducing thermal resistance between the fluid and the body, it has a number of drawbacks. First, it is very difficult to obtain large thermal exchange surfaces in an inexpensive manner due to the constraints imposed by the drilling. Moreover, the coupling between the channels made of aluminum and the connectors made of a different material, in the presence of water, induces galvanic corrosion of the aluminum, and this entails the risk that the corrosion products obstruct the channels for the cooling water. Aluminum corrosion moreover can etch and obstruct the channel in the case of non-treated cooling water and/or the occurrence of galvanic couples between different sections of the cooling circuit. Making anodized channels avoids the drawbacks related to aluminum corrosion yet it entails an increase in the complexity of the manufacturing process and, consequently, in the production costs, and in any case it does not allow obtaining large thermal exchange surfaces because it relies on the same geometry as the channels made by drilling.
In other prior art applications, cooling water ducts made of stainless steel are inserted into the pump body made of aluminum when the latter is manufactured. Insertion takes place by means of a co-melting process, i.e. the steel channel is introduced into the mold into which molten metal intended to form the body is poured. Such a solution allows obtaining large thermal exchange surfaces and avoids the corrosion problems. Yet, the co-melting process is complex and expensive. Moreover, such a process does not ensure a good thermal contact between the pump body and the ducts for the cooling fluid, due to the different thermal expansion coefficients and the different temperatures of the materials during cooling, which entails different shrinkages. Thus, notwithstanding the large thermal exchange surface, the yield of the thermal exchange is not always repeatable.
Further, in other prior art applications, cooling water tubes or channels made of copper or copper alloys are placed near the pump body made of aluminum and are secured thereto by interference driving, screwing or gluing. Even though such a solution exploits the high thermal conductivity of the tubes of copper or copper alloy, it is not free from problems of galvanic corrosion between copper and aluminum. Moreover, securing copper tubes to the pump body of aluminum entails technical problems or requires solutions that are expensive or are characterized by scarce process repeatability.
On the other hand, in the technology of heat exchangers with tubes and skirt, expansion by mandrel (“expansion”) of the tubes on the tube plates is utilized. Referring to FIG. 1, a mandrel 101 includes a rotating element 103, driven for instance by a pneumatic motor, causing the axial forward movement of a conical pin 105. The conical pin 105, while moving forward and rotating, pushes in radial direction a plurality of rollers 107 uniformly arranged along the axis of the mandrel 101, which at the same time is made to rotate by the pin 105. The radial and rotary motion of the rollers 107 causes a gradual enlargement of the tube in which the mandrel 101 is inserted, which enlargement is accompanied by a plastic deformation of the tube itself.
In the prior art, expansion is a commonly used operation when connecting the tubes of the heat exchangers to the tube plates. The tube plates are bored plates, which are placed along the tubes with a certain mutual spacing and through which the tubes pass, the plates having a much smaller thickness than the length of the tubes passing through them. In coupling the tubes of a heat exchanger with the tube plates, good quality of the contact between the tubes and the plates ensured by the expansion is sought for, so as to ensure a good robustness in tube positioning and the maintenance of a proper mutual position, eliminate the drawbacks related with vibrations, and also ensure hydraulic sealing between tubes and plates.
Yet, the prior art does not teach using expansion to improve the efficiency of thermal exchange between a metal body and a circuit for a heat exchange fluid inserted in the body for cooling/heating it.
Accordingly, there is a need for fabricating heat exchangers, particularly of the type which entail thermal exchange between a tube and a metal body. There is also a need for providing a method for improving the efficiency of thermal exchange between a metal body to be cooled/heated and a tube through which a cooling/heating fluid flows. There is also a need for providing a method for cooling/heating a metal body that avoids problems arising from the contact between the body and the cooling/heating fluid and from corrosion of the body due to the interposition of a tube of different material, without worsening the contact thermal resistance. There is also a need for providing fluid-tight connections between a heat exchanger and an associated circuit through which heat exchanging fluid flows to and/or from the heat exchanger.